You are correct. In English, when a noun is used to modify another noun (as an adjective does), it's referred to as a noun adjunct, attributive noun, or, more rarely, an adjectival noun (the last almost exclusively refers to a similar usage in Japanese). While it serves the purpose of an adjective, it's still technically a noun.

Examples are chicken soup, toy store, race car, and boat lane.

To be clear it's not about "spacefaring" billionaires but about "spacing" billionaires aka dumping them out an airlock into space as seen in various "The Expanse" scenes.